1. Field of the Invention
This invention relates to a filter for a camera, and more particularly, to a camera filter having an annular frame made of a synthetic resin and a filter plate held therein. This invention is also concerned with a method of making such a filter.
2. Description of the Prior Art
Camera filters are known having a frame made of a metal such as aluminum and brass. For instance, the filter shown in partial cross-section in FIG. 1 has an annular frame 1 provided with an axially projecting reduced diameter portion defining a flange 2 which attaches the filter to the lens holding tube of a camera. A filter plate 3 has its periphery held against the inner periphery of the flange 2, and a retaining ring 4 is screwed into the frame 1 to retain the filter plate 3 against the flange 2. Alternatively, the frame 1 is provided along its inner periphery with a dual diameter annular recess 5 including a larger diameter portion formed along the inner periphery of the flange 2, as is shown in FIG. 2. The filter plate 3 is held in the smaller diameter portion of the annular recess 5 between stub(s) or smaller diameter ring section 5a and retaining ring 6 of elastic material having a circular or other cross section is disposed in the larger diameter portion of the recess 5 to retain the filter plate 3 in position. Since the frames are made of a metal, however, these filters are heavy in weight, and are also expensive because the elements of the frame require machining. These filters are difficult to assemble automatically, and require a large amount of human labor for their fabrication. This necessarily adds to the cost of their manufacture.
In order to eliminate these disadvantages of filters having metallic frames, it has recently been proposed to make such frames of a synthetic resin. A filter having a frame made of a synthetic resin is light in weight, and less expensive, since the frame can be press-formed. A typical commercially available filter having a frame 1 formed from a synthetic resin is shown in FIG. 3. The flange 2 of the frame 1 is formed with an annular ridge 7 along its inner periphery. When the filter is assembled, the annular ridge 7 is elastically deformed to enable forcible insertion of a filter plate 3 into the frame. The filter shown in FIG. 3 is, however, unsatisfactory, since the filter plate fails to be tightly held within the frame, and also because the forcible insertion of the filter plate is likely to cause damage to the ridge 7, thereby impairing the appearance of the filter.
As shown in FIG. 4, it is also known to use a filter having a radially inwardly projecting annular flange 8 along the inner periphery of its frame 1, and a filter plate 3 held against the flange 8 and secured thereto and to the inner periphery of the frame 1 with an adhesive 9. The filter shown in FIG. 4 is unsatisfactory, too. In the event the filter plate is made of glass, the difference in the coefficient of expansion between the filter plate and the frame, which is made of a synthetic resin, is likely to cause detachment of the filter plate from the frame. Exposure of any excess adhesive from the area between the filter plate and the frame is often very detrimental to the overall appearance of the filter.
In any of the filters known in the art, the frame is made of a single element.